Experience:
To expound on what's been in other reviews, it fits huge. I'm 5'11 and 190 lbs with pretty broad shoulders, and I fit a medium perfectly. It's not tight, not baggy, just a decent fitted look. It still fits pretty long, like a tall-t (which I'm fine with, since I tuck into my pants as a base layer). Unless you're looking for gangsta steeze, size down.
It wicks nicely, has nice build quality, and is comfy. It's warm enough to layer with, but not so much that I can't see myself wearing it on its own on a milder (40+) day. It seems like it'll be much more durable than similar clothing items I've used.
It also has a non-stink antimicrobial finish, which seems to work, but those tend to wash out of synthetics... we'll see how that one holds up.
Only problem I have (and not for everyone) is that the thumb holes in the ribbed cuffs are pretty burly. If you have tighter glove fit, it might be a little uncomfortable.
In all, I really like it. It's a decent layering piece, I use it as an occasional work out shirt on cold days, and I even wear it to class. If you can get it at a discount, it's a great pick up.
P.S. the reason Gsrrr thinks a small would be too tight is because he's fat. Just FYI.

Pros: Thicker than other tech thermals. I usually wear this along with a base layer and it keeps me warm and dry in the socal weather. Has thumb holes but I dont use those.

Cons: Looooooong. I wear medium in almost everything so I ordered this in medium. The fit is good but the sleeves and overall length is just long. looks like I'm wearing a nightgown. If i were to order a small, I think it would have been too tight but the sleeve length and overall length would be better.

Experience:
Nice thermal overall. weird sleeve length and overall length. I guess if I had monkey arms it would fit better. =P Definately try before you buy.

Pros: Looks Great, Quality product and has thumb holes if thats a PRO. I bought the White and its great looking shirt

Cons: VERY BIG! I always buy XL shirts but decided to take a chance on this shirt and buy a Large. THat was a mistake, even the Large is way to big for me. The arms and length of shirt are way to long. Buy Smalll!